The traditional development process for a product, which exhibits the diagrammatic stages of drawing--model--model refinement--design--prototype--small--scale production--large--scale production is much too slow for the product cycles of industrial manufacture, which are becoming ever shorter.
Methods have therefore been developed in order to bring products on more quickly and to avoid intermediate handcrafted stages from a design drawing to a prototype or to small-scale production. These methods are frequently summarized under the terms of "rapid prototyping" or "fast free form manufacturing".
By contrast with shaping methods, such as milling or casting, in rapid prototyping systems, three-dimensional objects are automatically manufactured without the use of means such as molds, by controlled and locally limited chemical reactions (for example polymerization) or physical conversion (smelting/solidification) from preliminary stages of the respective material.
Some rapid prototyping systems are already in industrial use and may be briefly sketched.
1) Selective laser sintering--In the laser sintering method (A. Gebhardt, "Rapid prototyping: Werkzeug fur Schnelle Produktentwicklung" ["Rapid prototyping: a tool for rapid product development"], Hauser Verlag, Munich, Vienna 1996), pulverulent materials, usually plastics, are fused in layers by a laser to form an object. The powders used are preheated to just below the melting point of the material. The powders are fused (sintered) with one another under the action of the laser beam. The layer produced is then lowered by means of a suitable device, and a new layer of powder is applied to the already hardened layer and is in turn processed by the laser to produce the next object layer. The advantage of this technique is that it can be applied widely since, at least theoretically, all meltable pulverulent materials can be used. A disadvantage is the very rough surface of the objects produced and the fact that it is difficult to set the energy output of the laser. High laser energies do not lead to a sintering operation but to undesired complete melting of the powder grains, that is to say to a far reaching loss of the shaping properties. Excessively low energies particularly have the effect of fusing the layers to one another inadequately, with the result that the mechanical stability of the product produced is defective.
2) Thermal stereo lithography--U.S. Pat. Nos. 5,121,329 and 5,141,680 disclose methods in which a thermoplastic material is applied in layers by means of a nozzle. The material is applied, in a fashion capable of being shaped as a liquid or at least plastically, from the nozzle onto a support structure or onto already cooled layers, and solidifies in the desired shape. U.S. Pat. 4,665,492 describes a similar method; here, the liquefied material is applied by means of at least two independent particle canons. The extreme difficulty of controlling the resolution of detail is disadvantageous in these methods.
3) Stereolithography/photopolymerization--In methods of photo polymerization (A. Gebhardt, "Rapid prototyping: Werkzeug fur Schnelle Produktentwicklung" ["Rapid prototyping: A tool for rapid product development" ], Hauser Verlag, Munich, Vienna 1996), liquid monomers or oligomers are crosslinked to form a solid polymer under the action of UV radiation. The mostly free-radical polymerization can be initiated and terminated again by the decomposition of a photo initiator. Acrylate mixtures or epoxy resins are used as monomers, and UV lasers or UV lamps with a mask stop are used as light sources.
The object is also built up in layers. After one layer has been produced photochemically and polymerized out, new monomer is applied to the existing layer and induced to polymerize by radiation. The application of the new monomer layer can, for example, be performed by lowering the cured layer in a storage tank of the monomer. This method permits three-dimensional objects with complex cavities to be produced. Supplying energy by a laser is a problem, since the injected energy depends on the depth of penetration, that is to say the surface energy of the laser, the optical properties of the monomer mixture and the wavelength of the laser radiation employed.
The rapid prototyping methods described in the prior art have the common disadvantage that it is difficult to control the supply of energy, whether for melting powder particles or for initiating chemical reactions. This control is particularly problematic in the case of relatively large layer thicknesses, since the local energy intensity depends strongly on the depth of penetration. Relatively large local energy intensities such as are required for relatively large depths of penetration are, however, only connected with a relatively high thermal loading, or loading due to radiation chemistry, of the material employed. Higher energy intensities or a corresponding lengthening of the time over which energy is supplied are frequently not desirable with regard to thermally unstable materials.